Implement an "Send e-mail" flow node type

[GabrielMajeri]

One very common operation in business processes is the sending of (notification) e-mails.

Node inputs (customizable fields)

• From field (string): will be a hardcoded/fixed address for now, eventually it will be configurable through the admin/flow creator interface.
• To field / Destination e-mail addresses (string or array of strings)
• Subject line: a string, can use string interpolation and include parts of the node inputs
• Body: the message of the e-mail. Will be a plain text string, possibly with new lines, tab characters, emojis and interpolated strings.